Final Report Summary - LOCDIS (Safety in numbers or beware thy neighbour: collective motion and disease transmission in a migratory pest)
A key principle in the evolutionary ecology of disease is the degree to which hosts invest in defence against parasites and pathogens. It is generally accepted that disease risk is a function of host density and that risk of disease leads to selection on the host to minimize the potential costs of disease. A clear prediction that arises from these concepts is that hosts living at high densities should invest more in defence than those living at low densities. However, the Australian Plague Locust (APL), Chortoicetes terminifera, has been suggested to exhibit the opposite approach, with certain key immune functions shown to be lower in gregarious than in the solitary insects. This project attempted to determine why certain species appear to have evolved contrary behavioural and immunological strategies to combat disease transmission. We investigated three main objectives: 1) to determine the spatial and temporal pattern of natural disease epizootics in APL populations, 2) to compare immune function and disease resistance in gregarious and solitary locusts, and 3) to quantify the impact of protein availability on immune function, host behaviour and disease resistance.
Objective1: During the first fieldwork season (November 2011-March 2012) locust outbreaks were sampled from six locations in South Australia. Broad range microbial surveying via 16S next-generation DNA sequencing was undertaken on these samples to identify the bacterial strains associated with field populations of locust. Further locust collections were undertaken at sites in South Australia in the second locust season (November 2012 – March 2013), where analysis of immune-function and nutritional state was undertaken on individual samples collected.
Objective 2: After undertaking “proof of concept” assays to ensure a sound methodology, survival assays were undertaken to test mortality differences to a fungus, Metarhizium acridum. A dose-response methodology was undertaken, with 4 treatment groups (locusts reared in solitary conditions, locusts reared in gregarious conditions; locusts reared in solitary and then switched to gregarious; and locusts reared in gregarious and switched to solitary). As well as a survival assay, immune function assays were undertaken using blood samples taken from locusts in the 4 treatment groups: protein content, haemocyte counts, antimicrobial activity (lysozyme assay) and prophenoloxidase activity.
Objective 3: Laboratory experiments were undertaken using locusts feeding upon one of 20 diets, to achieve a full nutritional landscape. Immune assays were undertaken on these insects and the full nutritional content of the individuals recorded.
Our main findings were that dietary protein influenced constitutive immune function to a greater extent than did carbohydrate, indicating higher protein costs of mounting an immune defence than carbohydrate or overall energy costs. However, it appeared that increased immune function, as a result of greater protein ingestion, was not sufficient to protect locusts from fungal disease. We found that locusts restricted to diets high in protein (P) and low in carbohydrate (C) were more likely to die of a fungal infection than those restricted to diets with a low P:C ratio. We hypothesised that the fungus was more efficient at exploiting protein in the insect’s haemolymph than the host was at producing immune effectors, tipping the balance in favour of the pathogen on high-protein diets. When allowed free-choice, survivors of a fungus-challenge chose a less-protein-rich diet than locusts succumbing to infection and those not challenged with fungus.
In field populations, we found that body-condition varied greatly between individual locusts and between populations. Moreover, our measures of immune function were correlated to either the level of the haemolymph protein-pool (in the case of haemocyte density and lysozyme-like antimicrobial activity) or both the protein-pool and the body lipid content (in the case of prophenoloxidase activity): with implications for the role of biological pesticides in the control of locust populations. This research contributes to the knowledge-base in global food security and nutrition biology. The information gained throughout this project will give us insight into the nutritional and behavioural physiology of natural locust populations, thereby providing stakeholders with more tools to combat pest outbreaks.
Objective1: During the first fieldwork season (November 2011-March 2012) locust outbreaks were sampled from six locations in South Australia. Broad range microbial surveying via 16S next-generation DNA sequencing was undertaken on these samples to identify the bacterial strains associated with field populations of locust. Further locust collections were undertaken at sites in South Australia in the second locust season (November 2012 – March 2013), where analysis of immune-function and nutritional state was undertaken on individual samples collected.
Objective 2: After undertaking “proof of concept” assays to ensure a sound methodology, survival assays were undertaken to test mortality differences to a fungus, Metarhizium acridum. A dose-response methodology was undertaken, with 4 treatment groups (locusts reared in solitary conditions, locusts reared in gregarious conditions; locusts reared in solitary and then switched to gregarious; and locusts reared in gregarious and switched to solitary). As well as a survival assay, immune function assays were undertaken using blood samples taken from locusts in the 4 treatment groups: protein content, haemocyte counts, antimicrobial activity (lysozyme assay) and prophenoloxidase activity.
Objective 3: Laboratory experiments were undertaken using locusts feeding upon one of 20 diets, to achieve a full nutritional landscape. Immune assays were undertaken on these insects and the full nutritional content of the individuals recorded.
Our main findings were that dietary protein influenced constitutive immune function to a greater extent than did carbohydrate, indicating higher protein costs of mounting an immune defence than carbohydrate or overall energy costs. However, it appeared that increased immune function, as a result of greater protein ingestion, was not sufficient to protect locusts from fungal disease. We found that locusts restricted to diets high in protein (P) and low in carbohydrate (C) were more likely to die of a fungal infection than those restricted to diets with a low P:C ratio. We hypothesised that the fungus was more efficient at exploiting protein in the insect’s haemolymph than the host was at producing immune effectors, tipping the balance in favour of the pathogen on high-protein diets. When allowed free-choice, survivors of a fungus-challenge chose a less-protein-rich diet than locusts succumbing to infection and those not challenged with fungus.
In field populations, we found that body-condition varied greatly between individual locusts and between populations. Moreover, our measures of immune function were correlated to either the level of the haemolymph protein-pool (in the case of haemocyte density and lysozyme-like antimicrobial activity) or both the protein-pool and the body lipid content (in the case of prophenoloxidase activity): with implications for the role of biological pesticides in the control of locust populations. This research contributes to the knowledge-base in global food security and nutrition biology. The information gained throughout this project will give us insight into the nutritional and behavioural physiology of natural locust populations, thereby providing stakeholders with more tools to combat pest outbreaks.
